This invention relates generally to high-performance magnetometers and analog-to-digital (A/D) converters and, more particularly, to techniques for A/D conversion employing superconducting Josephson junction devices. There is a requirement for high-performance A/D conversion equipment in both military and commercial applications. Performance criteria include sensitivity, dynamic range, and sampling rate. High sensitivity instruments, such as magnetometers and gradiometers, employing superconducting devices typically suffer from limitations in their signal bandwidth, their ability to handle rapid slew rates of incoming signals, and their dynamic range. As will be further explained, the present invention overcomes these disadvantages.
A/D conversion using Josephson junctions has been described in the technical literature. John P. Hurrell et al. describe one such technique in a paper entitled "Analog-to-Digital Conversion with Unlatched SQUID's", published in the IEEE Transactions on Electron Devices, Vol. ED-27, No. 10, pp. 1887-96 (October 1980). SQUID is an acronym for Superconducting Quantum Interference Device.
The theory of operation of SQUID's for use in A/D conversion is explained in detail in the Hurrell et al. paper, and only a simplified explanation will be provided here. Similarly, the theory of operation of Josephson junctions is now widely known, and has been the subject of discussion in many technical papers. For example, see B. D. Josephson, "Supercurrents through Barriers", Advan. Phys., Vol. 14, pp. 419-51 (1965), and other papers cited in the Hurrell et al. paper.
A Josephson junction has a current-voltage characteristic that includes a region in which the current increases rapidly from zero, with practically no corresponding increase in voltage across the device. A SQUID is a circuit including one or more Josephson junctions and one or more inductive loads. A single-junction SQUID includes a Josephson junction connected across an inductance. If a current is injected into one end of the inductance and the other end is grounded, the resulting characteristics provide the basis for A/D conversion, as explained in detail in the Hurrell et al. paper.
The most pertinent property of the single-junction SQUID, from the standpoint of A/D conversion, is the relationship between the magnetic flux in the SQUID and the value of the injected current. This flux-current relationship is a periodic function and, depending on the circuit parameters chosen, a multi-valued function. The most significant aspect of the relationship is that the flux changes by a small quantum whenever the current increases by a small and precisely repeatable increment. This quantum of flux gives rise to a small but measurable voltage pulse across the junction. When the current is decreased, a flux quantum of opposite polarity is produced for each precise decrement of current, and a corresponding voltage pulse of opposite polarity is produced across the junction.
This property of the single-quantum SQUID forms the basis for the A/D converter described in the Hurrell et al. paper. A signal to be converted from analog to digital form is introduced into the single-junction SQUID as a varying current. Each time the current decreases by a predetermined increment, a measurable voltage pulse is generated across the junction. In this manner, the single-junction SQUID functions as a quantizer. The resultant pulses are then detected and counted in one or more counters. The principal advantage of the arrangement is its near perfect linearity. Another advantage is its sensitivity. The current increment, which determines the resolution, can be made extremely small. The flux quantum is only 2.07.times.10.sup.-15 weber, and the current increment is given by this value divided by the value of the load inductance (measured in henries).
Although the periodic nature of the currentflux relationship results in a highly accurate device, the basic mode of operation described suffers from two important limitations. One is that, if the input signal slews rapidly in value, it may not be possible to detect each of the flux transitions that occur. The other is that the sensitivity of the device in its basic form is limited to one flux quantum. Attempts to obtain sensitivity less than one quantum have resulted in a very much limited dynamic range. One attempt to provide finer sensitivity involves the use of a feedback signal to maintain a stable operating point on the flux-current curve. Basically, this is accomplished by applying a high-frequency "dither" current to the Josephson device and sensing any resulting output signals at the dither frequency. A feedback loop is used to drive the high-frequency output component to zero, a condition that will be obtained only if the operating point on the flux-current curve is at the desired location. In effect, the feedback signal provides an indication of the degree to which the input signal being converted differs from a signal that would have left the operating point undisturbed. However, this arrangement still suffers from limitations in slew rate and dynamic range. In addition, the approach requires the use of analog amplifiers or integrators, which may detract from the accuracy of the device.
A number of patents disclose superconducting A/D converters of various designs. U.S. Pat. No. 3,983,419 to Fang discloses a SQUID that can be used as either a sample-and-hold device or as a pulse generator coupled to an analog signal. The resulting signals can then be used in an A/D converter, the details of which are not disclosed. U.S. Pat. No. 3,458,735 to Fiske discloses a system for employing a plurality of Josephson junctions for A/D conversion. U.S. Pat. No. 4,315,255 to Harris et al. discloses an A/D converter using multiple SQUID's of the voltage latching type.
Another patent disclosing an A/D converter using Josephson junctions U.S. Pat. No. 3,949,395 to Klein. The Klein device uses multiple, voltage latching Josephson junctions as comparison devices in a successive approximation technique of conversion.
It will be appreciated from the foregoing that there is still a need for improvement in the field of superconducting A/D converters. The ideal converter should have high sensitivity and speed, but also a large dynamic range and the ability to accept input signals with rapid slew rates. The present invention is directed to these ends.